1. Field of the Invention
In the manufacture of printed circuit boards by the subtractive process, the process involves adhering a photopolymerizable film to the surface of a copper-clad laminate while covering the photopolymerizable layer with a protective film or support. Upon exposure to light in accordance with a predetermined pattern, those portions of the photopolymerizable layer exposed to the light form a solvent resistant pattern, while the unexposed portions may be readily washed away. The exposed copper layer may then be etched or plated.
The dry, photopolymerizable photoresist films are typically light in color and transparent. When viewed in relation to the underlying copper surface, it is extremely difficult to be able to determine the presence or absence of the film, so as to assure that the circuit design has been accurately delineated on the copper plate. In order to aid in the inspection of the photoresist layer, a number of different dye systems have been taught for incorporation in the photopolymerizable layer.
There are many considerations, both as to operability and practicality, in the choice of the dye systems. The dye system should allow for a clearly observable layer when in contact with the copper plate. Furthermore, the dye system may not significantly interfere with the curing of the layer upon exposure to light. The dye may interfere by absorbing light, so as to prevent light penetration through the layer or acting as a free-radical polymerization inhibitor, so as to react with free-radicals to terminate chains. In addition, the dye system should not be unduly sensitive to heat, base or acid, so that it may remain stable for long periods of time during storage, as well as during processing. Otherwise, if the dye has been subject to extensive degradation prior to curing of the photoresist, the observed color will be faint and not readily discernible.
2. Description of the Prior Art
U.S. Pat. No. 3,113,024 is directed to photosystems based on leuco bases of triphenylmethane dyes and teaches the heat sensitivity of the dyes with certain active sulfonyl halides.
U.S. Pat. No. 3,525,616 is directed to reproduction materials comprising at least one leuco triarylmethane dye, at least one N-vinyl carbazole, and at least one halogen hydrocarbon which splits off halogen when exposed to light. In the prior art disclosed system the halogen hydrocarbon activates both the color formation and the photopolymerization of the N-vinyl carbazole. In addition, many of the prior art halogen hydrocarbons, e.g., hexachlorocyclohexane, are highly toxic, can be absorbed through the skin and are strong irritants to the eyes. Moreover, some of the halogen hydrocarbons react with copper foil to form interfering complexes.
U.S. Pat. No. 3,769,023 is directed to light-sensitive reproduction material comprising (1) a hydroxyalkyl cellulose; (2) an ethylenically unsaturated vinyl monomer, preferably an N-vinyl monomer; (3) at least one compound which produces free radicals on exposure to light; (4) color formers taken from the general class of intermediates which produce color on exposure to condensation agents, oxidizing agents, and/or acids; (5) organic sulfur compounds for the promotion of adhesion; and (6) agents for improving the shelf stability of the product. In this system the compound which produces free-radicals activates both the color formation and the photopolymerization. The preferred activators are the organic halogen compounds in which at least three halogen atoms are attached to a single carbon atom.
U.S. Pat. No. 4,065,315 is directed to phototropic dye systems. It has an extended discussion of dyes and discloses the combination of an organic halide capable of liberating halogen free radicals on exposure to actinic light in combination with the free base of triarylmethane dyestuffs. The free base of triarylmethane dyestuff may be shown as ##STR1## One problem which attends the use of such dyestuffs is their instability to storage, a problem noted by the inventors in column 19, lines 60-62. Another problem attending the use of the dye system disclosed in U.S. Pat. No. 4,065,315 is the high toxicity of some of the organic halides used therein.